«جاءني زيد بل عمرو»
Zayd came to me—rather, ʿAmr.
«ما جاءني زيد بل عمرو»
Zayd did not come to me; rather, ʿAmr did.
Bal (بل) changes the direction of the report, often correcting the initially supplied identification. In the relevant context, the speaker abandons Zayd as the intended entrant and identifies ʿAmr instead.
Lā, by contrast, keeps the affirmative report about Zayd and excludes ʿAmr from it. Bal and lā thus perform different operations, even when both are used in a context of correcting a mistaken hearer.